Field of the Invention
The present invention relates to an idle speed control device for engine in which an idle speed is feedback-controlled such that an engine speed is equal to a target idle speed in idling, warming up of an engine or an increase in temperature of an exhaust gas purification catalyzer is promoted by largely retarding an ignition timing in cold starting, and a roughness control for stabilizing a change in torque caused by retarding the ignition timing is performed.
In general, in an automobile engine, an ignition timing, an air-by-fuel ratio, and the like are changed such that an engine speed is equal to a target idle speed in idling, so that the idle speed is feedback-controlled (for example, see Japanese Examined Patent Publication No. 7-92037). Here, the target idle speed is set to be an appropriate value according to drive conditions (for example, a water temperature of the engine, an operation of an air conditioner, and the like) to prevent engine stop from occurring and to prevent a fuel from being unnecessarily consumed. In the engine disclosed in Japanese Examined Patent Publication No. 7-92037, an idle speed is feedback-controlled by changing an ignition timing, and the feedback control variable of the ignition timing is adjusted depending on an intake air filling variable.
On the other hand, in an automobile engine, in order to purify an exhaust gas, a catalytic converter having an exhaust gas purification catalyzer is arranged on an exhaust passage. As the exhaust gas purification catalyzer, a conventional catalytic converter rhodium for simultaneously purifying HC, CO, and NOx is popularly used. The catalytic converter rhodium has the following characteristic features. That is, catalytic activity at a low temperature is relatively low, and exhaust gas purification performance cannot be sufficiently achieved. Therefore, in a catalytic converter using a catalytic converter rhodium as an exhaust gas purification catalyzer, when the engine is started in a cold state (in cold starting), an exhaust gas temperature is rapidly increased after the engine is started, so that an increase in temperature (activation) of the exhaust gas purification catalyzer must be promoted. A countermeasure in which an electric heater for heating the exhaust gas purification catalyzer is arranged to promote the increase in temperature of the exhaust gas purification catalyzer may be used. In this manner, an increase in cost and an increase in power consumption may disadvantageously occur.
In a general automobile, when an engine is started in a cold state, in view of that fuel consumption performance is improved, the engine is preferably started after the engine temperature (water temperature of the engine) becomes high to some extent. Therefore, in the automobile engine, the engine temperature must be rapidly increased.
For this reason, an engine system in which an increase in engine temperature or exhaust gas temperature is promoted by largely retarding an ignition timing in cold starting of the engine is provided (for example, see Japanese Unexamined Patent Publication No. 8-218995). More specifically, when the ignition timing is largely retarded as described above, the rate of changing from heat energy generated by ignition and combustion of a fuel-air mixture in a combustion chamber decreases, so that heat energy transmitted to engine cooling water or an exhaust gas increases. Warming up of the engine is promoted, not only an increase in temperature of the exhaust gas but also an increase in temperature of the exhaust gas purification catalyzer are promoted.
However, when the ignition timing is largely retarded to warm up the engine or promote an increase in temperature of the exhaust gas purification catalyzer, an output torque of the engine decreases, and combustion performance becomes unstable. For this reason, in a low output state (small-load and low-speed state), for example, a change in torque (roughness) increase in idling, and smooth driving of the engine is degraded. Therefore, as in an engine system disclosed in, e.g., Japanese Unexamined Patent Publication No. 8-218995, in an engine system in which an ignition timing is largely retarded to rapidly increase an engine temperature or an exhaust gas temperature, a roughness control for controlling an output torque of the engine is performed such that a change in torque ranges within an allowable limit in a predetermined low-output region.
For example, in an engine in which an idle speed is feedback-controlled such that the torque is changed by changing an ignition timing, the ignition timing is largely retarded in cold starting, and the ignition timing is corrected to stabilize a change in torque caused by the retarded angle so as to perform a roughness control, since the feedback control of the idle speed and the roughness control have the ignition timing as control variables, both the feedback control and the roughness control must be harmonized with each other, or integrally recognized to make them efficient, disadvantageously.
The harmonization and integration of the two controls must be also performed when the feedback control of an idle speed and the roughness control are performed such that an air-by-fuel ratio as a control variable.